The most common technique for reinforcing concrete involves the inclusion of tendons of wire mesh or steel rods in a formed slab. Prestressed concrete has long been a popular construction material due to the reduced amount of concrete used in a slab, which significantly reduces the weight of the slab. Prestressing can be achieved by the use of two general techniques: post-tensioning and pretensioning. Pretensioning is achieved by using reinforcing tendons of high tensile strength in a pouring mold. The tendons are tensioned, and the liquid concrete is poured into the mold encasing the tendons. When the concrete is set and cured, the tension on the tendons is released. The tension of the prestressed tendons exerts a tensile force on the surrounding concrete, imparting to it a tensile strength vastly superior to that of ordinary, reinforced concrete.
Pretensioning requires the use of a strong mold to withstand the force of the tension on the tendons. Tensioning of the tendons is usually achieved by means of anchoring one end of the tendon to the mold and applying force to the other end of the tendon by means of a device such as a hydraulic jack. When optimum force is attained, the unanchored end is anchored to the mold. Such procedures generally require a great degree of care and skill in preparation and are generally practiced in factory casting.
Post-tensioning is performed after the concrete slab is set and cured. The cables or rods, treated to prevent the concrete from adhering to them, are tensioned and anchored to the outer surface of the concrete slab. Post-tensioning is a simple and inexpensive means of prestressing concrete, and may easily be carried out on site. However, it suffers from creep of the tendons, resulting in loss of tension in the tendons.
The scale and cost associated with most prestressing of concrete precludes its use by most homeowners or handimen on small scale projects. The strength available from prestressed concrete may be used on a small scale for various home and garden projects.
There exists a need, therefore, for a method of preparing prestressed concrete that does not result in a loss of tension in the tendons and that is easy to perform, on site, by people, such as the home handyman, who do not possess a high degree of skill or expensive equipment. It is also desirable that the means of attaching the slabs together can also be performed without a high degree of skill or the need for expensive equipment.